In scanning SQUID microscopes, the output from a small Superconducting Quantum Interference Device (SQUID) is recorded as a sample is moved relative to the SQUID. The SQUID acts as the approximately point-like near field detector of the magnetic field, and the resulting data is used to construct an image of magnetic fields from the sample. The image of magnetic fields is further converted into the images of currents flowing in the sample under study by standard magnetical inverse.
The SQUID consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. The device may be configured as a magnetometer to detect extremely small magnetic fields—small enough to measure the magnetic fields of living organisms.
The great sensitivity of the SQUID devices is associated with measuring changes in magnetic fields associated with one flux quantum. One of the discoveries associated with Josephson junction was that flux is quantized in units
                              Φ          0                =                                            2              ⁢              π              ⁢                                                          ⁢              h                                      2              ⁢              e                                ≅                      2.0678            ×                          10                              -                15                                      ⁢                          tesla              ·                              m                2                                                                        (        1        )            
If a constant biasing current is maintained in the SQUID device, the measured voltage oscillates with the changes in phase at the two junctions, which depends upon the change in the magnetic flux. Counting the oscillations permits evaluation of the flux change which has occurred.
One of the main commercial applications of SQUID microscopes is fault detection of importance in the semiconductor industry. Images of the source currents can be generated by applying a magnetic inverse technique to the magnetic field images. Disadvantageously, with the present technology, the bandwidth of SQUID microscopes has not exceeded a few MHz. This is not an inherent limitation of the SQUID itself, whose bandwidth is many order of magnitude higher but rather of the electronics used to monitor the SQUID.
The present SQUID electronics consists of a feedback loop which maintains the magnetic flux through the SQUID loop constant. The electronics is based around an internal oscillator which operates typically at 100–300 KHz. The closed loop gain actually drops off somewhat lower than this value. Since the present invention of computer processors operate at over a GHz, well above the bandwidth of the present SQUID microscopes, the fault detection in the semiconductor industry with the present SQUID microscopes is somewhat limited for traditional scanning SQUID microscopes.
With the advance of semiconductor processor clock speed in excess of a GHz, the ability to image over a much larger bandwidth is desirable.
Therefore, the scanning SQUID microscopes with extended bandwidths to and above a GHz region is desirable and needed in magnetic microscopy.